keith@ux.acs.umn.edu (Keith MaloneyHuss) (02/06/90)
A while back, someone described a circuit that activated a recorder or other device when a phone was "off hook". Sorry, I can't remember who posted this, but my question is: When recording from the line using this circuit, I get a recording of a local radio station on top of the conversation being recorded. Is there any way to remove this radio interference? Basically, the recording tap goes through two 0.01 uF caps and two resistors from line to microphone input. The phone line is not shielded, but I never hear the radio interference on the phone itself. Thanks --keith
ee5391aa@hydra.unm.edu (Duke McMullan n5gax) (02/07/90)
In article <1067@ux.acs.umn.edu> keith@ux.acs.umn.edu (Keith MaloneyHuss) writes: >A while back, someone described a circuit that activated a recorder >or other device when a phone was "off hook". Sorry, I can't remember >who posted this, but my question is: >When recording from the line using this circuit, I get a recording of >a local radio station on top of the conversation being recorded. Is there >any way to remove this radio interference? Basically, the recording tap >goes through two 0.01 uF caps and two resistors from line to microphone >input. The phone line is not shielded, but I never hear the radio >interference on the phone itself. Keith, this is probably due to the RF being rectified by junctions in the recorder. To swallow that RF, shunt the phone line with another capacitor -- no larger than 1 nF, and probably more like 100 pF, on the line side of your interface circuit. Additionally, it may prove necessary to ground the "ground" side of your recorder input circuit. The phone line will be dc-isolated by the 10 nF caps that you're already using, so that should be no problem. Still, be careful. I've not seen any tape recorders in the past ten years that have a "hot" (from the pwr. line) side...they all use power transformers these days... but Murphy being the way he is, put a voltmeter on there first, just to pro- tect your fuses. ;^) If it's still persistent, try installing a couple of RF chokes in series with your two 10 nF caps. If that doesn't work, gather wolfbane and mistletoe, wait until the next full moon and.... Hope it helps, d "Don't reinvent the wheel -- steal the plans instead." - Don Lancaster Duke McMullan n5gax nss13429r phon505-255-4642 ee5391aa@hydra.unm.edu
elliott@optilink.UUCP (Paul Elliott x225) (02/08/90)
In article <1508@ariel.unm.edu>, ee5391aa@hydra.unm.edu (Duke McMullan n5gax) writes: > In article <1067@ux.acs.umn.edu> keith@ux.acs.umn.edu (Keith MaloneyHuss) > writes: > >When recording from the line using this circuit, I get a recording of > >a local radio station on top of the conversation being recorded. Is there > >any way to remove this radio interference? Basically, the recording tap > >goes through two 0.01 uF caps and two resistors from line to microphone > >input. The phone line is not shielded, but I never hear the radio > >interference on the phone itself. > > Keith, this is probably due to the RF being rectified by junctions in the > recorder. To swallow that RF, shunt the phone line with another capacitor -- > no larger than 1 nF, and probably more like 100 pF, on the line side of your > interface circuit. Additionally, it may prove necessary to ground the > "ground" side of your recorder input circuit. The phone line will be dc- > isolated by the 10 nF caps that you're already using, so that should be no > problem. Still, be [...] > If it's still persistent, try installing a couple of RF chokes in series with > your two 10 nF caps. One thing to remember about phone lines is that they are balanced. Thus even a large interfering signal (such as a radio station, 60Hz power line, or the 20 Hz ringing signal in an adjacent line) will show up as a common-mode signal and be ignored by the phone. (A brief intermission while I define some terms) balanced line: a two-wire line where each wire carries equal amplitude and opposite-phase (differential) signals. Both wires are "hot". example: twisted-pair. differential signal: see above. Termed a "metallic" signal in telephony. common-mode signal: in a balanced system, a signal that is in-phase on each wire. Most interfering signals are picked up equally by both wires, so will appear as common-mode signals. Termed "longitudinal" signals in phone-speak. (end of intermission) The best way to eliminate the interfering signal is to maintain the balance. The phone monitor circuit described above should be connected to a differential input on the tape recorder. Of course, most recorders only provide single-ended (unbalanced) inputs, so the next best thing is to put a coupling transformer between the monitor circuit and the recorder input. The series resistors need to be big to avoid loading the line, fortunately the signal levels are much greater than typical microphone levels, so we can afford to waste a lot of signal in the isolation network. You might try using a 600 Ohm 1:1 transformer and 22k resistors. This will give about 1 mV input to the recorder for a -20 dBm phone signal. Put a 600 Ohm resistor across the recorder-side of the transformer to preserve a flat frequency response. The 22k resistors probably won't meet phone company standards, but should work well enough for all practical purposes. You can try un-loading the transformer for increased signal, or using a higher impedance transformer. With 22k resistors and the 10 nF capacitors mentioned above, the low-frequency cutoff will be about 700 Hz. This is pretty high; perhaps using 47k resistors will give an adequate signal level while providing a better frequency response. A simpler method is to float the tape recorder. Most cassette recorders are not grounded; the power transformer has no ground connection. This probably is already the case, you definitely do _not_ want to earth-ground either of the recorder inputs. As suggested, an R.F. bypass cap of 1000 pF or so across the mic input and/or series inductors would help. You also can try a ferrite toroid as an RF common-mode choke: wrap the tap-pair through the toroid several times. Note that while the power-transformer doesn't provide a ground for audio signals, there will still be a path for R.F. signals through the transformer to the power lines. The series inductors and common-mode choke will help here. So will battery operation. -- Paul M. Elliott Optilink Corporation (707) 795-9444 {pyramid,pixar,tekbspa}!optilink!elliott "The dog ate my disclaimer."